Traditional conversion of UF.sub.6 involves one or more hydrolysis and reduction reactions utilizing oxygen and hydrogen bearing compounds. Experience shows that the conversion pathways are not straightforward and involve multiple intermediate uranium compounds. Hence, one step conversions involving multiple reactants, as shown by one of the following familiar reactions, are unlikely: EQU UF.sub.6 +3H.sub.2 +O.sub.2 .fwdarw.UO.sub.2 +6HF (1)
and EQU UF.sub.6 +2H.sub.2 O+H.sub.2 .fwdarw.UO.sub.2 +6HF (2)
The most commonly observed uranium intermediates generated by traditional conversion technologies are UF.sub.4 and UO.sub.2 F.sub.2 as shown by the following pairs of reactions: EQU UF.sub.6 +H.sub.2 .fwdarw.UF.sub.4 +2HF (3) EQU UF.sub.4 +2H.sub.2 O.fwdarw.UO.sub.2 +4HF (4)
and EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (5) EQU UO.sub.2 F.sub.2 +H.sub.2 .fwdarw.UO.sub.2 +2HF (6)
or EQU UO.sub.2 F.sub.2 +H.sub.2 O.fwdarw.1/3U.sub.3 O.sub.8 +1/6O.sub.2 +2HF (7)
Other reaction scenarios include UF.sub.5, UOF.sub.2, and UOF.sub.4. Uranium intermediates that the overall efficiency of the conversion process and largely determine the overall requirements and conditions for achieving the final uranium oxide product. For example, reactions (3) and (5) are rapid and unambiguous. On the other hand, reactions (4), (6), and (7) are traditionally slow and difficult to drive to completion without higher temperatures and large stoichiometric excesses of H.sub.2 and H.sub.2 O. It is well known by those skilled in the art that uranium intermediates also largely establish the physical properties of the final uranium oxide product.
The conversion of UF.sub.6 to UO.sub.2 has generally been pursued mainly by companies in the nuclear fuel business. For the most part, existing processes emphasize low-to-moderate operating temperatures with equipment designed to prepare chemically reactive UO.sub.2 suitable for a specific applications, i.e., the production of ceramic-grade UO.sub.2 for the fabrication of nuclear fuel. In most cases, uranium intermediates are deliberately formed via reactions (3) and (5), above, at relatively low temperatures, i.e., 200.degree.-300.degree. C. and the intermediates subsequently reacted in a second reactor via reactions (4) and (6), above, at moderate temperatures, i.e., 500.degree.-700.degree. C. These processes require large excesses of water and hydrogen to drive the reaction to completion. Moreover, even with the large stoichiometric excesses, extended processing times--on the order of hours--are required to bring about a reasonably high conversion efficiency of the intermediate to final product.
Representative of the numerous U.S. patents directed toward processes for the dry conversion of UF.sub.6 to uranium oxides is, for example, U.S. Pat. No. 4,830,841 and the U.S. patents listed therein. These patents describe processes for converting UF.sub.6 to uranium dioxide in furnaces, rotary kilns, fluidized beds or the like.
It is noted that U.S. Pat. No. 4,830,841 is concerned with a multiple step process for preparing UO.sub.2 from UF.sub.6 by reacting UF.sub.6 with steam to produce submicron uranyl fluoride powder, fluidizing a bed of uranium oxide material with a mixture of steam, hydrogen, and inert gas at about 580.degree. C. to about 700.degree. C. Thereafter, the submicron uranyl fluoride powder is introduced into the fluidized bed of uranium oxide material so that the uranyl fluoride powder is agglomerated, densified, fluidized, defluorinated, and reduced to a fluoride-containing uranium oxide material which is removed from the fluidized bed. Lastly, the fluoride-containing uranium oxide material is then contacted with additional hydrogen and steam at elevated temperature to obtain UO.sub.2 essentially free of fluorine.
In another prior art process, described in U.S. Pat. No. 3,260,575 entitled "Single Step Process for Preparation of Uranium Dioxide from Uranium Hexafluoride," a single step thermochemical process for the direct conversion of UF.sub.6 to a high density refractory UO.sub.2 fuel is described. This process requires reacting the UF.sub.6 with H.sub.2 O and H.sub.2 O at a very low pressure (.about.10 torr) on a hot surface, typically an Al.sub.2 O.sub.3 tube, maintained at 1200.degree.-1500.degree. C. in a resistance heated furnace to produce UO.sub.2. While this process deposited UO.sub.2 product as sub-micron powder, dendritic crystallites, and solid approaching theoretical density, it required extremely large excesses of water (20-30 times stoichiometric). Reduced pressure, diluent gases, and controlled feed configurations were required to prevent condensation of undesired intermediate uranium solids. Only UO.sub.2 was produced and the physical properties of the solids were not controlled. Also, it was difficult to recover the solids from the experimental system from the inside of the ceramic tube where they were formed. See also Nuclear Applications 1 (6), pp 584-588, December 1965.
In another process, a single fluidized bed reactor was demonstrated for the preparation of dense spherical UO.sub.2 particles from UF.sub.6 utilizing a cyclic mode to perform two discreet chemical unit operations. Unlike the low-temperature hydrolysis process for converting UF.sub.6 to UO.sub.2, the conversion reactor was operated at higher temperatures, e.g., 650.degree.-700.degree. C. to avoid the formation of undesirable UO.sub.2 F.sub.2, forming uranium products of UF.sub.4 and U.sub.3 O.sub.8. Thereafter, the intermediates were reacted with additional quantities of steam and H.sub.2 (with the UF.sub.6 flow discontinued) to form only UO.sub.2. Variation of the steam/UF.sub.6 molar ratio was observed to produce changes in the relative proportions of UF.sub.4 and U.sub.3 O.sub.8 formed during the UF.sub.6 feed period. While the cyclic process is simpler than the traditional UF.sub.6 conversion approaches, it is not normally efficient and is not generally preferred over a continuous process. See ANL-6902, I. E. Knudsen, et al, December, 1964.
None of these prior art processes or technologies provide for a single dry process that directly produces super-dense UO.sub.2, U.sub.3 O.sub.8, or UO.sub.3 and allows for the production of uranium oxide particulates of variable physical properties to enhance the industrial viability of the uranium oxide product or make a more environmentally stable uranium product for long term storage or permanent disposal. It is therefore a primary object of the present invention to provide such a process. Other objects of the present invention will become apparent to a person skilled in the art from a reading of the following preferred embodiments and appended claims and by reference to the accompanying drawing described hereinafter.